A.O. Smith GCVL-40 Technical Documents | Page 23

A.O. Smith Water Heater Technical Documents - GCVL-40.
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Residential Standard Gas Water Heater Service Handbook • 23

SUPPLEMENTS

Table 5.   

Minimum Free Area of Permanent 
Openings for VenƟ laƟ on 

and 

CombusƟ on Air Supply – All Air from 
Outdoors Only.

Based on the total BTU/HR input 
raƟ ng for all gas burning appliances 
within a confi ned space.

Opening Source

Minimum Free Area 

Per Opening (sq. in.)

*Direct to outdoors

1 sq. in. per 
4,000 BTU/HR 
(see Figure 25)

Vertical ducts

1 sq. in. per 
4,000 BTU/HR 
(see Figure 26)

Horizontal ducts

1 sq. in. per 
2,000 BTU/HR 
(see Figure 27)

Single Opening

1 sq. in. per 
3,000 BTU/HR 
(see Figure 28)

*These openings connect directly 
with the outdoors through a 
ventilated attic, a ventilated crawl 
space, or through an outside wall.  

B5:  Check that air source is clean 
and free of chemicals

Air for combustion and ventilation 
must be clean and free of corrosive or 
flammable chemicals. A failure due to 
corrosive chemicals in the air is not 
covered by the warranty.  Combustion air 
must be free of acid-forming chemicals 
such as sulfur, fluorine, and chlorine.  Be 
sure that air at the vent inlets is free of 
such chemicals.

B6:  Check that combustion 
ventilation is adequate

To confirm that your installation has 
enough combustion air, conduct the 
vent draft test on page 11 when 
installation is finished.

CombusƟ on Air Supply OpƟ ons

Direct to outdoors openings

Figure 25. 

Vertical duct openings

Figure 26. 

Horizontal duct openings

Figure 27. 

 

Single opening

Figure 28. 

 ATTIC  INSTALLATIONS  1

WARNING! 

• In all cases, a water heater must be 

installed according to its installation 
manual.  

• Consult the local codes of your 

area for specific ventilation and 
combustion air requirements.  In the 
absence of local codes, follow the 
National Fuel Gas Code (ANSI Z223.1-
current edition).  

• Failure to follow these warnings 

can lead to death, serious injury, or 
property damage.

Overview
Attic installations are common in 
some parts of the country.  In most 
cases, such installations are trouble 
free.  However, problems may arise 
from poor ventilation, high ambient 
temperatures, water leaks, and other 
potential hazards.  Such issues include 
pilot outage, water damage to the 
living spaces below, and elevated 
water temperatures.  Even so, these 
problems can be addressed.  Some of 
the potential solutions are listed below.

Pilot Outage

Pilot outage is more common in attic 
spaces and hot environments than 
in other installations.  The following 
factors can contribute to pilot outage, 
either individually or in combination:

•  Elevated air temperature

• Insufficient makeup air (air needed 

for the proper ventilation of 
combustion gases) 

•  Blocked air screen or flame arrestor

•  Improper venting of the water heater

• Decompression

• Wind

Originally published as “Issues 

Related to Gas Water Heaters Installed in Attics 
(Atmospheric Vent)” 
(324841-000). 

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